This invention relates in general to systems for measuring surface characteristics of samples such as semiconductors, and in particular, to such a system with self-calibrating capability.
Spectrophotometers and ellipsometers have been used for measuring surface characteristics such as film thickness and refractive indices of single or multilayer films on substrates such as semiconductors. Materials that are commonly found on semiconductors include oxides, nitrides, polysilicon, titanium and titanium-nitride. Ellipsometers can utilize a single wavelength or broadband light source, a polarizer, a modulator, an analyzer and at least one intensity detector. In this type of conventional ellipsometer, the light from the light source is modulated and sensed by the detector. The detector signal is analyzed to calculate the ellipsometric parameters. This type of ellipsometer is described for example in U.S. Pat. No. 5,608,526.
Ellipsometric measurements are affected by the environment such as temperature changes and mechanical vibrations. For this purpose, ellipsometers are calibrated periodically to account for such environmental effects. Reference samples with known thicknesses and optical characteristics have been used during calibration. However, with the continual downsizing of semiconductor devices, ultra-sensitive ellipsometers have been developed that can measure film layers with thicknesses of the order of angstroms. These systems require reference samples having thin films for accurate calibration. When such thin film reference samples are used, even minimal oxidation or contamination is significant and may result in significant calibration errors. It is therefore desirable to provide an improved surface optical measurement system such as an ellipsometer with better calibration characteristics.
In International Application No. PCT/US98/11562, a stable wavelength calibration ellipsometer is used to precisely determine the thickness of a film on a reference sample. The measured results from the calibration ellipsometer are used to calibrate other optical measurement devices in the thin film optical measurement system. However, this requires the reference sample to be calibrated by means of the calibration ellipsometer each time the thin film optical measurement system is to be used for measurement so that this procedure may be cumbersome. Furthermore, the characteristics of a film or films on the reference sample may have changed between the time of calibration and the time of the measurement, especially where not every measurement is taken immediately after the calibration process.
U.S. Pat. No. 5,416,588 proposes another approach where sufficiently small phase modulation (usually on the order of 3 or 4xc2x0) are applied by means of a photo-elastic modulator (PEM). By limiting its phase modulation to several degrees, the detectable signal is proportionally reduced so that the signal-to-noise ratio of the scheme in U.S. Pat. No. 5,416,588 may be less than desirable for a number of applications. By using only small phase modulation, the amount of information obtained concerning the parameters of the measurement system itself will be limited, so that it may be impossible to characterize all of the important system parameters in some systems.
None of the above-described systems are completely satisfactory. It is therefore desirable to provide an ellipsometer with improved calibration characteristics in which the above-described difficulties are not present. It is especially desirable to provide an ellipsometer that has self-calibration capability.
An ellipsometer with self-calibrating capability is proposed. Instead of having to calibrate the ellipsometer system parameters that may change over time or due to environmental factors, they are derived together with the ellipsometric parameters from the data measured by the ellipsometer. Therefore, there is no need for reference samples or for calibration ellipsometers. All the user needs to do is to derive the system parameters together with the ellipsometric parameters so that any alteration in the system parameters that affect the accuracy of measurement of the ellipsometric parameters may be taken into account. Since the system parameters can be derived from the same data from which the ellipsometric parameters are derived, any change in the system parameters can be accounted for exactly, without having to assume that the system parameters have stayed the same between a calibration process and a measurement process. The invention is also not restricted to small phase modulations. Therefore, the signal-to-noise ratio of the instrument will be adequate for self-calibration in a wide variety of systems and applications.
In the preferred embodiment, a beam of radiation having a linearly polarized component is supplied to the sample. Radiation from the beam that has been modified by the sample is detected. The polarization of the beam of radiation is modulated prior to its detection and one or more ellipsometric parameters of the sample and one or more parameters of a system used in the above process are derived without restrictions as to the magnitude of modulation.
In conventional ellipsometers, essentially unpolarized radiation is provided by the light source to a polarizer to polarize the radiation before it is applied to the sample and radiation from the polarized beam is passed to an analyzer after modification by the sample before the radiation is applied to the detector. In the conventional scheme either the polarizer or the analyzer is rotated but not both. As an improved design in a related aspect of the invention, a beam of radiation is passed through a first rotating polarizer before the beam is applied to the sample. Radiation from the beam after modification by the sample is also modulated by a second rotating polarizer to provide the modulated beam. Radiation from the modulated beam is detected by a detector. From the detector output, one or more ellipsometric parameters of the sample may be obtained. Preferably, system parameters as well as the one or more ellipsometric parameters are derived from the detected radiation to self-calibrate the system and to improve the accuracy of the measurement. Also preferably the beam of radiation is passed through a fixed polarizer between the radiation source and the detector.
As yet another improved design, radiation from a beam having a polarized component is supplied to the sample. Radiation from the beam that has been modulated by the sample is detected. Radiation from the beam is modulated before or after its modification by the sample but before its detection by means of a rotating polarizing element. The modulated radiation that is detected is also passed through a fixed linear polarizer prior to its detection. One or more ellipsometric parameters of the sample may then be derived from the detected radiation.
Another factor that affects the accuracy of measurements in ellipsometers is sample tilt or change of focus due to variations in the heights of the samples. In conventional ellipsometry, the optical paths used for detecting the accuracy of focusing and sample tilt are separate from those used for ellipsometric measurement. This results in errors or instability due to drift or misalignment between the two subsystems. This invention contemplates that a portion of the radiation directed towards the detector is diverted to a position sensitive detector for detecting sample tilt or inaccuracy in focusing due to factors such as changes in sample height. This feature may be used in ellipsometry as well as other surface optical measurement systems such as spectrophotometry.
Semiconductor manufacturing frequently reserves on a wafer a small electrical contact pad which can be used for ellipsometric measurements, where the area frequently have square shapes. The illumination beam in ellipsometry is typically directed at an oblique angle to the sample. Therefore, if the illumination beam has a circular cross-section, the resulting illuminated spot on the sample will be elliptical in shape. Since the size of the square pads reserved for ellipsometry on semiconductors may be small in size, it may be difficult to fit the elliptical spot within such pads. By using a cylindrical objective to focus the illumination beam onto the sample, this would have the effect of flattening the elliptical spot to better fit within the confines of the pads. Preferably, the cylindrical objective focuses the illumination beam to a spot which is substantially circular in shape.
The above described ellipsometer may be advantageously used together with another optical instrument for measuring samples. Preferably, the outputs of the ellipsometer and of the other optical instrument may be used to derive sample information as well as parameters of the ellipsometer to improve accuracy of measurement. In one application, the combined system may be used to measure film thickness information of the sample and depolarization of radiation caused by the sample. The depolarization derived may indicate sample characteristics such as surface roughness.
Alternatively, each of various configurations of the ellipsometer may by itself be used for measuring film thickness information and depolarization caused by the sample, with or without also deriving systems parameters of the ellipsometer from the same measurement output.